The present invention relates to a permanent-magnet synchronous machine which is designed for three-phase operation.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
A typical three-phase permanent-magnet synchronous machine (with permanent magnet excitation) has a stator in which each tooth is surrounded by a winding and which has a number of holes q=½, as well as a rotor, on which permanent magnets are arranged with a pole gap in the circumferential direction and with a skew with respect to the axial direction. The permanent-magnet synchronous machine has torque ripples which are caused in particular by the fifth and seventh harmonics of the stator and rotor fields. The number of holes corresponds hereby to the number of stator slots N1 divided by three times the number of the poles 2p, i.e. q=N1/(3×2p). This therefore results, for example, in a torque ripple when the fifth harmonic of the rotor field is coupled to the fifth harmonic of the stator field, and the rotation speed of the two fields is different to the rotor rotation speed.
A large pole gap between the magnet poles is beneficial in order to simplify the manufacturing technology. However, the harmonics are particularly high, and therefore also the oscillating torques caused by them, when the pole gaps are between 20% and 42.9% of the pole pitch τp. Until now, scarcely any machines have therefore been produced with a number of holes q=½, which have pole gaps in this range.
With regard to the torque ripple, measures have generally been taken until now to reduce or eliminate the fifth and seventh harmonics of the rotor. This can be realized, for example, by a targeted pole coverage of the rotor with magnets and/or skewing or staggering of the magnets. International patent document WO 2004/109894 A1 discloses a permanent-magnet synchronous motor in which the fifth harmonic of the rotor field is cancelled out by skewing through half the slot pitch 0.5×Tn or by skewing by ⅗×Tn, wherein Tn refers to the slot pitch. The seventh harmonic of the rotor field is cancelled out by a skew of 3/7×Tn. The combination of this skew of 60% of a slot pitch Tn with a pole coverage of 85% leads both to complete damping or cancellation of the fifth harmonic and of the seventh harmonic. The combination of the skew of 3/7×Tn with a pole coverage of about 80% also leads both to complete damping or cancellation of the fifth harmonic and of the seventh harmonic.
It would be desirable and advantageous to provide an improved three-phase permanent-magnet synchronous machine to obviate prior art shortcomings and to extend their design freedom.